Miniaturized optical systems are desirable in many fields of use, including telecommunications, medicine, and space exploration. A typical miniaturized optical system comprises several passive optical devices, such as lenses, mirrors, switches, diffraction gratings, etc., and possibly even one or more active optical devices, such as lasers or light-emitting diodes. These optical devices are arranged on the surface of a small substrate in precise alignment to one another.
Miniature optical systems have been formed using fabrication techniques borrowed from MEMS (Micro-Electro-Mechanical Systems) technology. In particular, surface micromachining, deep reactive ion etching (RIE), and crystallographic-dependent wet etching have been used, individually, to fabricate integrated optical devices such as mirrors, gratings, lenses, and optical switches.
Whichever of these techniques is used to fabricate optical systems, it is important that the resulting optical surfaces (i.e., surfaces that interacts with light beams) have little or no surface roughness, have controlled surface shape, and are mechanically-stable over time and during temperature changes. Also, to offer substantial utility, the fabrication technique should be capable of producing something more than trivial arrangements of simple optical devices. And therein lie the problems.
As applied to the manufacture of miniature optical systems, surface micromachining, deep RIE, and crystallographic-dependent wet etching have certain drawbacks. These techniques, and their drawbacks, are described below.
Surface micromachining creates a three-dimensional micromechanical device for use as an optical element via successive depositions and etches of thin sacrificial and structural films on a substrate. The etches pattern or “cookie-cut” the intended shape of the device into the various films. The nature of this deposition-based approach results in devices that usually consist of a number of very thin plates (i.e., a few microns thick).
For a variety of reasons, the characteristic multi-plate structure of a surface-micromachined device exhibits mechanical instability. Of equal or greater concern for optical applications is that the surface of the plates is typically very rough, and therefore exhibits undesirable loss and scattering of optical energy.
The second fabrication technique mentioned above, deep RIE, creates features, such as holes and channels, which are very deep and narrow. In some optical systems, the sidewalls of these channels are used as mirrors or parts of other optical devices. Advantageously, the position and orientation of deep RIE features are independent of the crystal planes (described further below) in the substrate. It is possible, therefore, to form complicated arrangements of optical devices using deep RIE.
Unfortunately, the sidewalls of deep RIE features are rough due to the nature of the etching process. Like surface-micromachined optical devices, the rough sidewalls scatter optical energy in undesired directions, leading to loss of optical energy and potential cross-talk with other optical devices. Furthermore, the sidewalls are not perfectly vertical nor are they flat. As a result, an optical beam that is incident on a deep RIE sidewall will be reflected out-of-plane, relative to the inbound signal, and/or distorted, which are usually very undesirable results.
By way of background to the third fabrication technique, atoms in a crystalline solid are in a very ordered arrangement, called a “crystalline lattice.” Within the lattice, the atoms align in various planes, each with a characteristic arrangement of atoms. These planes are referred to as “crystal planes.”
The orientation of the top surface of silicon with respect to its crystal structure is used to define the type of silicon used and the orientation of etched features. For example, <100> silicon has a top surface that is one of its <100> crystal planes and <110> silicon has a top surface that is one of its <110> crystal planes. The numerical notation that is used to identify the crystal planes, which is referred to as a Miller Indice, is a symbolic vector representation for the orientation of the crystal or atomic plane in the crystal lattice.
The third fabrication technique mentioned above, crystallographic-dependent wet etching, produces features that are formed from specific crystal planes, as are contained in a single-crystal material.
In the case of silicon, for example, several etchants exist that etch non-<111> crystallographic directions faster than <111> crystallographic directions. This enables the formation of structures having features that are nearly perfectly aligned to <111> crystal planes and that are nearly perfectly planar as well.
Unlike deep RIE, perfectly vertical side-walls (with respect to a perfectly horizontal top surface) can be formed using a crystallographic-dependent etch (e.g., an etchant that comprises potassium hydroxide, etc.) on a <110> silicon wafer. Unfortunately, these vertical side-walls are necessarily aligned to a subset of the <111> crystal planes. As a consequence, crystallographic-dependent etching has a rather limited ability to form complex miniaturized optical systems.
It is apparent, then, that there are drawbacks to the use of existing techniques for the fabrication of miniature optical systems. For systems that contain only optical devices, surface micromachining and deep RIE are not suitable because they create poor optical surfaces. While crystallographic-dependent etches provide good optical surfaces, they offer a limited ability to create complex arrangements due to their dependence on the fixed spatial relationships of crystal planes.
The situation is even worse for more complex optical systems that include both optical devices and micromechanical devices. At least one of these devices will be compromised based on the choice of fabrication method. It would be desirable to use more than one of these fabrication methods to form complex optical arrangements, but this has historically been unworkable. In particular, once either deep RIE or crystallographic-dependent etching has been used to form one of the devices—the optical device or the MEMS device—there is great difficulty in repeating that technique or using another technique to form another device. This difficulty arises due to the severe topographical variations in the surface that result from using the first technique to form the first device. Specifically, once a surface exhibits such severe topography, it is extremely difficult to add a mask layer using conventional photolithography. Similarly, once surface micromachining has been used to form a micromechanical device on the surface of a substrate, there is great difficulty in using either deep RIE or crystallographic-dependent etching to form another device. This difficulty arises from an incompatibility of material properties and/or etch selectivity.